The long-term goal of this project is to elucidate the cellular mechanisms of parathyroid hormone receptor (PTHR) action in bone. It has generally been assumed that internalized receptors are constitutively recycled. We discovered that intact PTH peptides promote receptor recycling, whereas amino-truncated PTH peptides, which accumulate in renal failure, lead to PTHR ubiquitination and downregulation. This is the first demonstration of PTHR ubiquitination, a reversible post-translational modification. The mechanism underlying ligand-regulated ubiquitination and deubiquitination is unknown. This gap in our understanding is a major focus of the proposed studies. PTHR recycling begins soon after its sequestration but is not complete for some time. Preliminary studies now show that a cytoplasmic PDZ protein, SNX27, accelerates receptor recycling without affecting signaling or internalization. The proposed studies are designed to define the sustainability of the parathyroid hormone receptor, which after activation and internalization is destined either for degradation or recycling. The actions and mechanism by which PTH receptor salvage is regulated are poorly understood. The unifying hypothesis of this proposal is that post-translational modifications and cytoplasmic adapter chaperones determine PTHR degradation and recycling. Two interrelated aims are developed to test this theory. Aim 1 will characterize novel PTHR post-translational modifications, their mechanism of action, and effect on receptor fate and function. Aim 2 addresses the mechanism by which cytoplasmic chaperone proteins facilitate PTHR recycling. The planned studies use an array of complementary biophysical, molecular and biochemical, and cell biological tools. These studies will characterize a novel mechanism to explain regulated PTHR recycling. The results will generate new information that is relevant to understanding bone turnover. The outcomes will help define potential therapeutic targets for treating PTH resistance, osteoporosis and other metabolic bone diseases.